This project is directed at identifying and differentiating heritable influences on vocal development in primates. Current work involving comparisons of the behavior of the "gothic arch" subtype of squirrel monkey in Costa Rica with captive social groups of the same subtype originating from South America, has identified vocal characteristics common to both Costa Rican and South American groups, as well as other vocal attributes found only in the Costa Rican population. Other current work has analyzed the development of the isolation call of infant common marmoset twins. While acoustically similar to the cell given by separated adults, the infant isolation call is simpler consisting of a steady tone lasting about 1 second. Twins separated from their parents will call together on nearly the same pitch, producing a unique acoustic signal that is readily identified and distinguishable from the isolation calls of either twin alone. Analysis of the isolation calls from the adult members of our marmoset colony reveal that each adult is very stable in its calling behavior over weekly 15 min. separations. Related work has analyzed the temporal fine structure inherent in the serial production of calls by separated marmosets. Both common and pygmy marmosets produce isolation calls that are grouped together in a sequence of 2-10 closely spaced units. Analysis reveals an orderly relationship between the sequence position of each unit, its duration, and the time interval between it and adjacent units in the same series. In both marmoset species, there is a significant positive correlation between call duration and interval to the preceding unit. However, in the pygmy marmoset intervals and durations increase with sequence position, whereas in the common marmoset the opposite rule is followed. This is the first demonstration in any nonhuman primate of a rule of temporal ordering in a complex vocal sequence, and it suggests a fine degree of genetic programming in regulating in regulating the vocal output of these species.